The invention relates to spatial light modulators (SLMs) and, more specifically, to a vanadium oxide (VO) compound based SLM for the infrared (IR) range. (Note, the term VO is used herein throughout, including in the claims, as an acronym to refer to a vanadium oxide compound or compounds, in general; VO is not used herein as a chemical symbol referring only to vanadium monoxide.)
SLMs are real-time reconfigurable devices capable of modifying the amplitude (or intensity) of an optical wavefront as a function of spatial position. Electrical addressing of individual pixels in the SLM controls the reflection/transmission characteristics in the array. Applications of SLMs include scene simulation, dynamic spatial frequency filtering, analog multiplication and optical correlation.
There are very few options for SLMs within the mid and far IR. Mechanical modulators exist; however, they are fragile, very expensive, and capable only of imparting a phase change to the beam of light. Such devices are not usable in an image plane.
The only practical SLMs for the IR appear to be those using the vanadium oxides with the most useful being vanadium dioxide (VO.sub.2). The VO compounds are heat-activated semiconductors with significantly different band gaps for different phases. Their characteristics are such that they act as dielectrics below a threshold temperature and conductors above this temperature.
The SLM designs described and claimed herein exploit the temperature dependent reflection/transmission characteristics of vanadium oxide (VO) thin films, i.e., the reflection increases as the temperature increases and the vanadium oxide becomes a narrow band gap semiconductor. When the temperature decreases the VO thin film becomes transmissive.
VO thin films exhibit a temperature dependent band gap and hysteresis about a transition temperature of approximately 60.degree. C. This hysteresis is a result of a thermally induced atomic rearrangement in the VO.sub.2 's crystalline structure changing it from a dielectric to conductive state. As noted, accompanying this transition is a change from a state of low to high reflectivity in the IR band.
The transformation or switching time between reflective states is a function of both the array's thermal design and the rate of energy injection into the thin film. Measured switching times have approached 30 nanoseconds with atomic rearrangement times estimated at less than 10 picoseconds.
FIG. 1 shows a model hysteresis curve which approximates the reflective characteristics of a 1000 .ANG. VO.sub.2 thin film at a wavelength of 4 .mu.m. This hysteresis curve has a width of approximately 20.degree. C. with reflectivity from 20% to almost 100%. The hysteresis' width, reflective contrast and slope are functions of the thin film's deposition technique, doping and substrate material.
There are several factors guiding the design of an SLM thermal array. The first is that the design must lend itself to the fabrication of a large, high resolution SLM. Each of the thermal pixels must provide accurate temperature control to the VO thin film. Accurate thermal control permits full mobility about the hysteresis curve. Because the VO.sub.2 's reflective state is temperature dependent, care must be taken to limit thermal cross-talk between adjacent pixels as well as any temperature deviation across the array itself.
Other considerations include the temperature pulse itself, whose rise and fall times determine the maximum refresh rate of the SLM. Another consideration for the array design is the array substrate material. For the SLM to be used in the transmissive mode, the array substrate must have a high IR transmission so that the full reflective contrast of the VO compound is not partially masked. A reflective mode SLM requires that the array be either highly absorbing or transmissive. The most flexible SLM design will allow for use in either the reflective or transmissive mode. These and other considerations are met by the SLM embodiments described and claimed herein.